The invention relates to an apparatus for controlling a star wheel for the exact positioning of specimen containers which are disposed in N segments of the star wheel such that the specimen containers succeed one another around the circumference of the star wheel and are moved by the star wheel incrementally in succession to a desired stopping position, the apparatus further having a drive motor that acts upon the star wheel and is controlled to rotate in increments.
Such star wheels are used in measurement technology, for example, to carry a number of specimen containers, e.g., test tubes, chained together, for the performance of various processing steps at respective processing locations, and to enable large-scale automation of these processing steps, as disclosed in Federal Republic of Germany DE-OS No. 30 13 868. A prerequisite for such an automatic succession of processing steps, for example in performing a luminescence measurement, adding liquids by pipette, and so forth, is that the drive of the star wheel be controllable so exactly that precise positioning of each specimen container is possible relative to the apparatus by which such a method step is performed. Accordingly, the star wheel must be stopped at exactly defined angular intervals and at an exactly defined position of the specimen containers conveyed, for the duration of the particular processing or measurement operation. DE-OS No. 30 13 868 provides no information as to how this could be satisfactorily accomplished.
A comparable problem in the precise processing of work pieces can be found in Federal Republic of Germany DE-OS No. 34 47 728, and the apparatus shown there is for this purpose embodied such that on the shaft of the work piece to be positioned, a control disk rotates in synchronism with the work piece, and this control disk is scanned with the aid of a bifurcated light gate.
The disadvantages of this previously known apparatus are substantially that the friction among the various components and the resultant slip, or inexact assembly, can make it impossible to assure an unequivocally defined association of the scanned marking in the control disk to the position of the workpiece, which must be equally precisely defined. It is therefore impossible to apply this principle to the exact positioning of specimen containers.
It is also generally known to use a reflex light gate, as disclosed in the publication entitled "Eigenschaften und Anwendungen von Reflexlichtschranken" [Properties and Applications of Reflex Light Gates] by Dipl.-Ing. R. H. Dollwetzel, Munich, which appeared in Feinwerktechnik & Messtechnik [Precision Instruments and Measuring Instruments] 93, 1985, 7, pp. 369-371, FIG. 5), for instance for detecting the teeth of a gear wheel.
A comparable application of a reflex light gate is also disclosed in U.S. Pat. No. 4,204,115 for measuring the circumferential speed of the disk of an electrical power meter.
All of these disclosures relate to the monitoring of a continuously moving object by means of a reflex light gate; in the type of apparatus with which this invention is concerned, however, operation is discontinuous and incremental, and for exact positioning of the specimen containers dynamic influences, in particular inertia effects of the rotating masses, must also be taken into account.